The physical properties of respired gases and physiologic measurements of ventilation and respiratory gas exchange will be studied in neonates. An instrumentation system which is accurate, precise, sensitive to minor fluctuations, non-invasive and adaptable to a variety of clinical settings will be developed and evaluated for safety and efficacy first using a model simulating infant respiratory exchange. Respiratory heat and water vapor loss will be determined from the measurements and the influence of such variables as birth weight, gestational age, age from birth and respiratory disease requiring environmental oxygen supplementation and/or mechanical ventilatory support examined.